The present invention relates generally to the field of marine geophysical surveying. More particularly, in one or more embodiments, this invention relates to using a rigid-stem lead-in comprising a plurality of interconnected rigid stems in a marine geophysical survey.
Techniques for marine surveying include marine geophysical surveying, such as seismic surveying and EM surveying, in which geophysical data may be collected from below the Earth's surface. Geophysical surveying has applications in mineral and energy exploration and production to help identify locations of hydrocarbon-bearing formations. Certain types of marine geophysical surveying, such as seismic or electromagnetic surveying, may include towing an energy source at a selected depth in a body of water, typically above the seafloor. One or more geophysical sensor streamers also may be towed in the water at selected depths by the same or a different vessel. The streamers are essentially long cables having geophysical sensors disposed thereon at spaced-apart locations. A lead-in typically couples the sensor streamer to the survey vessel. Actuation of the energy source emits an energy field into the body of water. The energy field interacts with the rock formations below the water bottom with changes in the energy field due to this interaction detected by the geophysical sensors positioned on the streamers. The detected energy is used to infer certain properties of the subsurface rock, such as structure, mineral composition and fluid content, thereby providing information useful in the recovery of hydrocarbons.
Since the introduction of three-dimensional seismic surveying, there has been interest in towing wider and longer spreads of sensor streamers, which require more cables, larger lateral separation, deeper tows, and longer or bigger cables with more and more external equipment. To achieve the desired lateral spread between the sensor streamers, spreading devices have been used, which may include lateral depressors, such as inclined plates or wings. Some lateral depressors typically the larger one may be connected to the survey vessel using a separate tension member while others may be attached to the lead-in connecting the sensor streamer to the survey vessel. For towing sensor streamers, the lead-in can take the shape of an umbilical with or without fairing connected to one or more wings (also reformed to as depressors) for achievement of both lateral, vertical, or combined offset from the vessel trajectory and may be used in combination with weights, flotation devices, and sometimes active propulsion to achieve deep towing/large offsets.
When towing sensor streamers up to several kilometers in length and from 1 to 3 inches in diameter, as may be done in three-dimensional surveying, a tension of a little over 1 ton may be normally required at the industry standard of 5 knots transversal speed. The tension increases with increased speed. In order to keep the cables at as lateral spread of more than 1,000 meters, the tension often exceeds 10 tons on the outermost lateral depressor because it typically sees its own drag and the drag from the lead-in or tow wire in addition to the required lateral lift for the sensor streamer. The tension will typically be the highest in the outermost members and the surveys with the widest or largest spreads. For the purpose of storing these lead-ins and other tension members and to be able to deploy as much cable as desired while stopping at any position under tension, a high-torque and often brake-able winch may be used. In instances where the lateral depressor is not connected to a sensor streamer, as pure wire with high strength and smaller bend radius than for the lead-ins may be used for the tension member. In other instances, a steel, or Aramid-armored umbilical cable with copper and fiber fibers inside has been used.
However, these cables often traverse through the water with up to 45° of cross flow. Uneven water flow around the cables may produce alternating shedding forces which may cause transverse vibrations known as “strumming” or “vortex index vibrations” in the cables. Strumming may be problematic with lead-ins as the adding drag due to strumming results in higher load with corresponding lower lifting performance on the lead-ins. In addition, the turbulent flow within the water caused by the strumming generates acoustic noise that may interfere with data collection. Strumming may further generate stresses at equipment connection points and can accelerate equipment failure. A number of techniques have been developed to reduce problems associated with strumming as a cable is towed through the water. One technique involves attachment of fairings to the cables to reduce strumming in the water. There are number of different types of fairings in use, including hard fairings and hairy or fabric fairings. Hard fairings may include a streamlined shell or other structure attached to sections of the cable. Drawbacks to hard fairings may include increased complexity of the cable-handling system as a faired cable cannot be stored directly on a drum when large or in long lengths. Hairy or fabric fairings may include fairing hairs attached to the cable. While hair or fabric fairings may remove vibration, very little reduction in drag may be achieved as the reduced drag is typically compensated by the increased diameter/area of the faired cable.
Accordingly, there is a need for improved techniques for reducing drag forces which may reduce strumming noise interfering with data collection and increase towing efficiency.